1. Field of the Invention
The present invention relates to elastic wave devices, such as surface acoustic wave devices and boundary acoustic wave devices, and more particularly, to an elastic wave device in which an elongated section is provided in a portion of an electrode finger.
2. Description of the Related Art
Hitherto, elastic wave devices have been widely used as resonators or band pass filters. Japanese Unexamined Patent Application Publication No. 2011-101350 and International Publication No. WO2011/088904 each disclose a structure of a surface acoustic wave device in which transverse-mode spurious emissions are suppressed by the formation of a piston mode. For example, FIG. 9 of Japanese Unexamined Patent Application Publication No. 2011-101350 and FIG. 4 of International Publication No. WO2011/088904 show that elongated sections are provided in electrode fingers of an interdigital transducer (IDT) electrode. By providing these elongated sections, a low acoustic velocity region is formed.
FIG. 12 of Japanese Unexamined Patent Application Publication No. 2011-101350 and FIGS. 8(c) and 9 of International Publication No. WO2011/088904 show that a film covers part of an IDT electrode. More specifically, a film covers a region farther outward than a central region in the extending direction of the electrode fingers of the IDT electrode. Because of this structure, a low acoustic velocity region is formed.
However, in the structure in which elongated sections are provided in electrode fingers, there is a limitation on increasing the width of the elongated sections. That is, if the width of the elongated sections is excessively increased, the elongated sections may contact adjacent electrode fingers. Accordingly, the acoustic velocity of a low acoustic velocity region is not sufficiently decreased. It is thus difficult to reliably suppress transverse mode ripples. Further, additionally disposing a film on part of an IDT electrode increases the complexity of the manufacturing process steps and the cost.